This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-284146 filed on Sep. 27, 2002.
1. Field of the Invention
The present invention relates to a fuel injection pump and more particularly to a load timer mechanism of a fuel injection pump of a distributor type.
2. Description of Related Art
The emission regulation of diesel engines has become increasingly stringent due to the increasing environmental concern. In general, when the fuel injection timing of the diesel engine is retarded, the emission of NOx is reduced. However, when the fuel injection timing is retarded to reduce the emission of NOx, the engine power is disadvantageously reduced. To address such a problem, it is known to use of a load timer, which reduces the amount of timer advancement when the load of the engine is reduced to reduce the emission of NOx and noises without reducing the engine power under the high load condition where high engine power is required. Such a load timer is disclosed, for example, in Japanese Unexamined Patent Publication No. 57-119132.
In the prior art load timer, an orifice penetrates through a governor sleeve of a centrifugal governor, which is reciprocated according to the rotational speed of the engine. Furthermore, a fuel relief passage, which is communicated with a low pressure side of a fuel injection pump, is formed in a governor shaft, along which the governor sleeve is guided. Upon operation of the governor sleeve, when the orifice of the governor sleeve is communicated with the fuel relief passage of the governor shaft, fuel in a pump housing of the fuel injection pump is relieved into the low pressure side of the fuel injection pump through the orifice of the governor sleeve and the fuel relief passage of the governor shaft. When the fuel pressure in the pump housing is reduced, a timer piston of a hydraulic timer received in the pump housing is retarded. As a result, cam rollers, which are in rolling contact with a face cam of the fuel injection pump, are moved to retard the fuel injection timing.
However, in the prior art load timer, since the amount of timer advancement is reduced in the low load condition, misfire could occur to cause generation of white smoke emissions.
To address such a problem, according to the disclosure of Japanese Unexamined Patent Publication No. 57-119132, a solenoid valve, which opens and closes the fuel relief passage, is provided to limit timer retardation caused by the load timer under operating conditions where generation of white smoke is expected, for example, in a case of driving the vehicle at highlands or in a case of operating the engine at low coolant temperature conditions, such as at the time of engine cranking. However, due to the increasingly stringent emission regulation, the amount of emission of NOx needs to be further reduced. Thus, according to the disclosed technique, it is difficult to further improve, i.e., further increase the amount of timer retardation. That is, when the load timer is operated to further increase the timer retardation, the solenoid valve, which limits the timer retardation, is actuated. As a result, the amount of timer retardation is limited. In some cases, the load timer could become inoperable.
The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a fuel injection pump, which is capable of limiting the timer retardation in the low load range and the high load range and also capable of improving the timer retardation in the intermediate load range with a relatively simple structure.
To achieve the objective of the present invention, there is provided a fuel injection pump for an internal combustion engine. The fuel injection pump includes a pump housing, a hydraulic timer mechanism and a load timer mechanism. The pump housing defines a pump chamber therein. The hydraulic timer mechanism is received in the housing and adjusts fuel injection timing based on fuel pressure of the pump chamber. The load timer mechanism is received in the housing and adjusts the fuel injection timing based on a load of the engine. The load timer mechanism includes a governor shaft and a governor sleeve. The governor shaft is secured to the housing. The governor shaft includes a fuel relief passage and a first annular groove. The fuel relief passage is communicated with a low pressure side of the fuel injection pump and has an outer opening in an outer peripheral surface of the governor shaft. The first annular groove is recessed in the outer peripheral surface of the governor shaft such that the first annular groove is axially displaced from the outer opening of the fuel relief passage. The governor sleeve is axially slidably supported around the governor shaft and is moved back and forth relative to the governor shaft based on the load of the engine. The governor sleeve includes a port and a second annular groove. The port extends between an outer peripheral surface and an inner peripheral surface of the governor sleeve and includes an outer opening placed in the outer peripheral surface of the governor sleeve and an inner opening placed in the inner peripheral surface of the governor sleeve. The port conducts the fuel pressure of the pump chamber from the outer peripheral surface to the inner peripheral surface of the governor sleeve. The second annular groove is recessed in the inner peripheral surface of the governor sleeve such that the second annular groove is axially displaced from the port. When the governor sleeve is positioned within a predetermined axial range, the first annular groove and the second annular groove are at least partially overlapped one another to define a fuel pool therebetween. When the engine is in one of a full load condition and a high load condition, the first annular groove and the second annular groove are at least partially overlapped one another, and the fuel pool is not substantially communicated with the port to prevent substantial communication between the port and the fuel relief passage. When the engine is in one of a partial load condition and an intermediate load condition, the first annular groove and the second annular groove are at least partially overlapped one another, and the fuel pool is substantially communicated with the port to substantially communicate between the port and the fuel relief passage. When the engine in one of a no load condition and a low load condition, the first annular groove and the second annular groove are not substantially overlapped one another to prevent substantial communication between the port and the fuel relief passage.